Bone fixation system and bone screws having anti-back out feature

ABSTRACT

The system includes a plate with a screw hole and a bone screw wherein the screw and plate cooperate such that the screw is inhibited from back out from the plate after inserted into bone. Described embodiments include a pawl, with and without ratchet, an interfering clip about the screw head, and a snap-fit engagement between the screw and screw hole. The screw hole is preferably provided with openings at at least one of two diametric sides which provide dynamic compression functionality and access under the screw for an instrument to facilitate screw release.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates broadly to orthopedic devices. More particularly,this invention relates to systems for engaging bone screws relative tobone plates.

2. State of the Art

For various fractures of bones of the body, compressive plating is awell known technique to impart the stabilization desirable for properhealing. In compressive plating, a rigid, typically metal plate isplaced on the outer surface of the bone across the fracture, and screwsextend through the plate and are secured into the bone on either side ofthe fracture in a manner which permits the rigid plate to offer supportto the bone during healing. The screws include threads along a shaftadapted to engage cortical bone. Most commonly, the head portion of thescrew is a standard screw head which provides a compressive force abouta corresponding round screw hole of the plate as the fixator is threadedinto the bone, thereby causing compression of the plate against thebone.

U.S. Pat. No. Re. 28,841 to Allgower describes a plate that is used withgenerally standard bone screws having heads with a convex undersurface.The plate includes oblong screw holes which each define at one end anupper ramped portion and a generally smaller radius of curvature aboutthe ramped portion. In use, a hole is drilled into the bone through thescrew hole adjacent the ramp and a screw is inserted into the drilledhole and rotated until the head of the screw contacts the ramp. Uponsuch engagement, there is displacement of the bone plate in a directionto move the ramped portion away from the screw and to cause the plate toapply pressure to maintain the bone parts together about the fracture intight engagement. The holes in a such a plate are commonly referred toas dynamic compression holes (or DCH). However, micromotion between thebone and the portion of the screw within the bone can cause loosening ofthe entire assembly, diminishing the stability of the set fracture and aloss of compression across fracture.

More recently, threaded screws with heads which threadably engage inthreads in the plate to lock the screws relative to the plate have beenused. However, such systems do not provide the necessary control ofcompression between the plate and bone. Control over compressive forcesis lost as soon as the threads of the head of the screw lock relative tothe plate. Therefore, such a system provides sub-optimal stability forattachment of certain plates to bone. In addition, even such threadedengagement can loosen over time.

As a result, several systems have used secondary discrete lockingelements to lock a bone screw to the plate. For example, U.S. Pat. No.6,383,186 to Michelson teaches the use of a set screw which seatsagainst the head of the bone screw to prevent backing out of the bonescrew. U.S. Pat. No. 6,152,927 to Farris teaches a screw and washerassembly which provides compression against the head of the bone screwto lock the bone screw within the screw hole and prevent it from backingout. Both of these systems require that the surgeon work with separateand small locking elements at the time of the screw insertion, and suchsmall elements may easily become lost in the surgical wound.

U.S. Pat. No. 5,549,612 to Yapp et al. teaches a system in which a screwcan be locked relative to the plate with a cam permanently mounted in anaperture in the plate. The cam cannot provide any downward force againstthe screw head, thereby limiting potential fixation. As such, if the camrotates just a small amount from a locking angle, the fixation providedby the cam may be lost. Moreover, the shape of the cam (as shown inFIGS. 4 and 4A of the patent) suggests that the cam applies an upwardforce against the screw head which disadvantageously counters thecompressive force of the screw against the plate.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a plate and screwsystem whereby the amount of compression between the plate and bone canbe controlled completely by the surgeon.

It is another object of the invention to provide a plate and screwsystem which requires the application or activation of no additionalelements beyond the insertion of the bone screw to effect an anti-backout feature to the bone screw.

It is also an object of the invention to provide a plate and screwsystem which is adapted to provide displacement of a plate in adirection which applies pressure to maintain the bone parts together intight engagement about a fracture.

In accord with these objects, which will be discussed in detail below, abone plating fixation system includes a plate with one or more screwholes, a bone screw including a head, which in one embodiment includes asubstantially circumferential groove with a ring rotatably fixed withinthe groove. The ring has a small portion, or pawl, extending beyond thecircumferential surface of the head. The screw can be rotatably insertedinto the screw hole without significant resistance from the pawl as thepawl will deflect into the groove when rotated in the ‘insertion’direction. However, the configuration of the pawl creates significantinterference that resists the screw head from being rotated in adirection leading to disengagement of the screw from the underlyingbone, and the screw can only be disengaged with substantial manual forceapplied to the screw; i.e., not from the forces of micromotion betweenthe bone and screw shaft. The screw hole may be provided with verticalgrooves that function as a ratchet for the pawl of the ring. Inaddition, the screw hole may be provided with openings at at least oneof two diametric sides which provide (i) dynamic compression hole (DCH)functionality from either of two directions and (ii) access under thescrew for an instrument to facilitate screw release.

In another embodiment, the pawl is coupled to the plate and the screwhead is provided with one or more longitudinal grooves displaced aboutthe circumference. Rotation of the screw head causes the pawl to engagein one of the grooves in a manner which will not be overcome by theforces of micromotion between the bone and screw shaft, but which can beovercome by a surgeon using instrumentation.

In yet another embodiment, both of the screw head and screw hole includecircumferential grooves, and one of the screw head and hole include aring partially provided in the groove. Under compression, the ring canbe forced to be substantially completely within the groove of thecomponent to which it is coupled; i.e., the screw head or screw hole.When the screw is inserted into the screw hole, the ring is compressedby the other of the components, and then expands partially into thecorresponding groove of the other of the components, thereby providing asnap-fit between the screw and plate.

According to a further embodiment of the invention, the screw holeincludes a small lip or tabs about its upper entry which is slightlysmaller than the largest diameter across the screw head. When the screwis inserted by the surgeon, it is forcibly inserted past the lip ortabs. The force of micromotion will be insufficient to overcome theinterference between the screw head and lip or tabs, and thus the screwhead will be trapped within the screw hole. This embodiment is preferredfor use with DCH screw holes, as such provide the additionalfunctionality of access under the screw for an instrument to facilitatescrew release.

Additional objects and advantages of the invention will become apparentto those skilled in the art upon reference to the detailed descriptiontaken in conjunction with the provided figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective section view of a bone fixation plate providedwith a screw having a ring assembled thereto with an anti-back outstructure according to a first embodiment of the invention;

FIG. 2 is a plan view of the ring of FIG. 1;

FIG. 3 is a section view across line 3-3 in FIG. 2;

FIG. 4 is a top view of the screw of FIG. 1, indicating the groove forthe ring in broken lines;

FIG. 5 is a perspective broken section view of the plate of FIG. 1;

FIG. 6 is a top view of a bone fixation plate, wherein the screw holesprovides dynamic compression from either of two directions and twoaccess locations to facilitate removal of the screw;

FIG. 7 is a perspective section view across line 7-7 in FIG. 6;

FIG. 8 is a top view of a second embodiment of an anti-back out screwassembly;

FIG. 9 is a top view of a third embodiment of an anti-back out screwassembly;

FIG. 10 is a perspective view of a variable angle screw according to theinvention; and

FIG. 11 is a perspective section view of a fourth embodiment of ananti-back out screw assembly.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to FIG. 1, an assembly of a bone fixation plate 10 and screw12 according to the invention is shown. The plate 10 can be anyorthopedic plate which has application in providing compression or otherstabilization to bone, including but not limited to, plates forfractures of the diaphysis and/or metaphysis of long bones, plates forplacement on the mandible or other portions of the skull, and plates forosteosynthesis, particularly along the vertebrae. For example, U.S. Pat.No. 6,706,046 and U.S. Pub. Nos. 20050065524A1 and 20050182405A1, whichare hereby incorporated by reference herein in their entireties,describe plates which are suitable for use with the invention. The plategenerally includes a plurality of screw holes, one such screw hole 14shown in FIG. 1. The screw hole 14 is threadless and in a preferredembodiment includes a flared entry 18, an upper larger cylindricalportion 20, a central spherical portion 22, and a lower smallercylindrical portion 24. The cylindrical portions 20 and 24 alternativelymay be frustoconical in shape.

Referring to FIGS. 1 through 4, the screw 12 includes a head 30 with anupper recess 32, e.g., a hex slot, for a driver, a shaft 34 with boneengaging threads, and a conical taper 36 at the lower end of the head 30leading into the shaft 34. The head 30 includes upper and lowercylindrical portions 38, 40 and a central spherical portion 42 thatmatches the dimensions and radius of the bone screw hole 14. The head30, preferably at the spherical portion 42 includes a circumferentialgroove 44. The groove 44 preferably includes two diametrically opposedand tangential flats 46, 48 for rotatably fixing a ring 50 within thegroove. The ring 50 has a substantially circular outer circumferencewhich corresponds in diameter to the outer diameter of screw head 30surrounding the groove 44, and two internal flats 52, 54 correspondingto flats 46, 48. The ring 50 also has two ends 56, 58, one of whichdefines a resilient pawl 56 extending slightly beyond the outercircumference of the screw head 30 at the groove 44 (FIG. 4).

In use, a hole is preferably pre-drilled through the screw hole and intothe underlying bone at a location substantially concentric with thecircular center defined by the upper and lower cylindrical portions 20,24 of the bone screw hole 14 (FIG. 1). The head 30 of the screw 12 seatswithin the space defined by portions 20, 22, 24, and as head 30 isdriven toward the bone the plate is compressed against the bone. Thebone screw is maintained in a fixed angle (e.g., normal to the lowersurface of the plate) by being subject to three points of fixation. Thebone screw 12 can be driven until a desired compression is effected. Thepawl 56 fixes the level of compression and prevents any loosening thatmay occur through micromotion. That is, due to the preferred non-radialconfiguration of the pawl, the screw can be rotatably inserted into thescrew hole without significant resistance from the pawl, as the pawlpresents a ramped surface 57 and when the screw is rotated clockwise(i.e. in a direction in which the shaft threads are engaged into thebone) the pawl will preferably at least partially deflect into thegroove 44. At most, the pawl will tap a shallow thread into the screwhole. However, the configuration of the pawl 56 defines a sharp 59 whichcreates significant interference with the walls of the screw hole whenthe screw is attempted to be rotated counter-clockwise; i.e., in adirection in which the shaft threads are disengaged from the bone. Assuch, the pawl 56 resists unintentional backing out by the screw fromthe screw hole and such resistance can only be overcome with substantialand intentional manual force applied to the screw, and not from themicromotion between the bone and the screw shaft.

Turning now to FIG. 5, the screw hole 14 may be provided with one ormore vertical grooves 60, particularly in the spherically curved portion22, that function as a ratchet for the pawl 56 of the ring 50 (FIG. 2).Such structure provides a positive feedback for the surgeon to indicateadvances of the screw. The grooves 60 can be located at a specificangular displacement about the circumference, e.g., spaced 20° apart,which depending upon the pitch of the shaft thread will correspond to aspecific longitudinal advancement of the thread into bone and/orincremental compressive force between the plate and bone.

In addition, referring to FIGS. 6 and 7, the screw hole 14a may besimilar to hole 14 but provided with openings 66, 68 at at least one oftwo diametric sides which provide (i) DCH functionality from either oftwo directions and (ii) access under the screw head 30 for an instrumentto facilitate screw release after a screw is seated in the screw hole.More particularly, the openings 66, 68 at the diametric sides aredefined by a portion of the screw hole having an upper ramped surface 70and a lower smaller radius of curvature 72 than the lower cylindricalportion 24 (FIG. 1). Much like the dynamic compression screw holedescribed in U.S. Pat. No. Re. 28,841 to Allgower, which is herebyincorporated by reference in its entirety herein, such a screw holeallows the screw to apply force against either upper ramped surface 70to drive the plate in a direction transverse the axis of the screw. Byproviding ramped surfaces 70 at diametric locations about the screwhole, the plate may be driven in either of two directions depending uponscrew placement.

In accord with another mode of using the plate and the screw 12, a holeis drilled for the screw 12 along an axis normal to the plate which isoffset towards one of the ramped surfaces 70 and generally concentricwith one of the curves 72; i.e., away from the circular center definedby the upper and lower cylindrical portions 20, 24 (FIG. 1) of the screwhole 14a. The bone screw 12 is then driven into the hole until the lowerconical surface 36 the screw head contacts the ramp 70 and thus causesdisplacement of the plate 10 by the distance required to seat the head30 in the central concave spherical portion 22 (FIG. 1). Thisdisplacement applies pressure which maintains bone parts together abouta fracture in tight engagement. The screw 12 is tightened, with the pawl56 located to engage the central portion of the hole and resist rotationin an opposite disengaging direction.

In addition, if screw removal is necessary, the screw may be rotated(generally by no more than 90°) to position the pawl 56 within anopening 66, 68 to disengage the pawl from the screw and facilitate screwremoval. Moreover, the opening 66, 68 provides sufficient access for arelatively flat tipped instrument to be positioned against theundersurface of the screw head, e.g., at 74, to pry up the screw head ifscrew removal is necessary.

Turning now to FIG. 8, another embodiment of the invention is shown. Thepawl 156 is coupled to the plate 110. The pawl 156 is fixed at leastpartially within a slot 176, and may be spring-biased on spring 178 tomove longitudinally within the slot. The screw 112 includes a screw head130 provided with one or more longitudinal ratchet grooves 180 displacedabout its circumference. Rotation of the screw head 130 causes the pawl156 to engage in one of the grooves 180 in a manner which will not beovercome by the forces of micromotion, but which can be overcome by asurgeon using a driver inserted into the driver opening 132 of the screw112. Alternatively, the pawl may be retracted against the spring-bias toeliminate impediment to screw removal (as well as initial screwinsertion). In addition, the pawl 156 may be oriented at an angle αrelative to a radius L_(T) to the screw hole to facilitate screwinsertion, but to inhibit screw back out.

Referring to FIG. 9, another embodiment of the invention is shown. Thepawl is a is resilient spring steel member 256 which enters the screwhole and may enter grooves 180 in the screw head 130, but which iseasily deflected when the screw head 130 is rotated in a direction of ascrew insertion, but which resists screw rotation in disengagementdirection.

Turning to FIG. 10, the embodiments shown in FIGS. 8 and 9 may also beused with a variable angle screw 212 having a substantially completelyspherically curved screw head 230. Such a screw head 230 is preferablyprovided with one or more longitudinal grooves 280 for engagement with apawl or similar member. The grooves may be curved along meridians of thescrew head.

Turning now to FIG. 11, another embodiment is provided in which both thescrew head 330 and the screw hole 314 include circumferential grooves344, 345. A resilient C-ring 382 is substantially provided in the groove344 on the screw head 330 but is expanded slightly beyond thecircumferential surface of the screw head. When the screw is driven intothe screw hole 314, the groove is sufficiently deep to permit the ring382 to be substantially completely compressed within the groove 344 ofthe screw head. Once the screw 312 is driven so that the screw head andscrew hole grooves 344, 345 meet, the ring 382 expands outward into thegroove 345 of the screw hole, thereby providing a snap fit engagementbetween the screw 312 and plate 310. The C-ring may alternatively be anO-ring. As yet another alternative, the ring may be initially fixedwithin the groove 345 of the screw hole, and then enter the groove 344on the screw head when the screw head is seated in the screw hole. Asdescribed above, the screw hole may also include access openings tofacilitate screw removal.

It is appreciated that the general shape of the screw hole in any of theembodiments described above permits the use of variable angle screwshaving spherically curved heads matching the curvature of the centralspherical portion. Such screws would be able to be directed at anysurgeon directed angle within a range of angles. In addition, suchscrews may also be provided with a pawl, grooves, or rings, as describedabove to prevent screw back out.

There have been described and illustrated herein embodiments of bonefixation systems and bone screws. While particular embodiments of theinvention have been described, it is not intended that the invention belimited thereto, as it is intended that the invention be as broad inscope as the art will allow and that the specification be read likewise.It will therefore be appreciated by those skilled in the art that screwholes and screws of other shapes and designs, while still provided withthe features of the invention, are within the scope of the invention.Other modifications can be made to the provided invention withoutdeviating from its scope as claimed.

1. A bone fixation system, comprising: a) a bone plate including a bonescrew hole; and b) a bone screw having a non-threaded head portion and ashaft portion with bone- engaging threads; and c) structure permittingsaid head portion of said screw to be rotated in said screw hole in abone engaging direction and which directly inhibits said head portion ofsaid screw to be rotated in said screw hole in a bone disengagingdirection.
 2. A bone fixation system according to claim 1, wherein: saidstructure is a pawl coupled to said plate.
 3. A bone fixation systemaccording to claim 2, wherein: said pawl is resilient.
 4. A bonefixation system according to claim 2, wherein: said pawl is directedinto said screw hole at an angle relative to a radius to said screwhole.
 5. A bone fixation system according to claim 2, wherein: said headportion includes at least one ratchet groove.
 6. A bone fixation systemaccording to claim 4, wherein: said head portion and said screw areconfigured for fixed angle orientation of said shaft portion of saidscrew relative to said plate.
 7. A bone fixation system according toclaim 4, wherein: said head portion and said screw are configured forvariable angle orientation of said shaft portion of said screw relativeto said plate.
 8. A bone fixation system according to claim 1, wherein:said structure is coupled to said screw head.
 9. A bone fixation systemaccording to claim 8, wherein: said structure is resiliently deformablerelative to said head portion.
 10. A bone fixation system according toclaim 8, wherein: said structure is rotationally fixed relative to saidhead portion.
 11. A bone fixation system according to claim 8, wherein:said screw hole includes at least one ratchet groove.
 12. A bonefixation system according to claim 1, wherein: said screw hole is adynamic compression hole.
 13. A bone fixation system according to claim1, wherein: said screw hole includes at least one access opening from anupper portion of said plate to a lower portion of said screw head whensaid screw head is seated in said screw hole.
 14. A bone fixationsystem, comprising: a) a bone plate including a bone screw hole; and b)a bone screw having a non-threaded head portion and a shaft portion withbone- engaging threads, wherein one of said plate and said head portioninclude a pawl which engages the other of said plate and said headportion to inhibit the bone screw from backing out the screw hole.
 15. Abone fixation system according to claim 14, wherein: said head portionincludes a circumferential groove, and a ring is provided is provided insaid groove, wherein said ring includes said pawl and said pawl extendsbeyond a circumference of said head portion.
 16. A bone fixation systemaccording to claim 15, wherein: said groove includes a non-circularportion and said ring includes a corresponding non-circular portion torotationally fix said ring within said groove.
 17. A bone fixationsystem according to claim 14, wherein: said pawl is coupled to saidplate and said screw head includes at least one longitudinal ratchetgroove displaced about its circumference, wherein rotation of said screwhead within said screw hole causes said pawl to engage one of said atleast one grooves.
 18. A bone fixation system, comprising: a) a boneplate including a non-threaded screw hole; b) a bone screw having anon-threaded head portion and a shaft portion with bone- engagingthreads, wherein at least one of said screw hole and said head portionincludes a circumferential groove; and c) an element provided in saidgroove and in contact with a side of said head portion which inhibitssaid bone screw from backing out of said screw hole.
 19. A bone fixationsystem according to claim 18, wherein: both of said screw hole and saidhead portion include said grooves, and said grooves are in alignmentwhen said head portion is fully seated in said screw hole.
 20. A bonefixation system according to claim 18, wherein: said element includes apartially compressive ring coupled in a groove on at least one of saidscrew hole and said head portion of said screw.
 21. A bone fixationsystem according to claim 18, wherein: said element is an expandablering which is compressible into said groove in said head portion duringinsertion of said head portion into said screw hole and which expandspartially into said groove in said screw hole when said screw head isfully seated.
 22. A bone fixation system according to claim 18, wherein:said element includes a ring in a groove on said screw head, said ringprovided with a pawl that substantially interferes with said screw holewhen said screw is rotated in a direction of disengagement from thebone.
 23. A bone screw for use with an orthopedic plate provided withnon-threaded screw holes, comprising: a shaft portion with bone-engagingthreads, and a non-threaded head portion including a pawl extending fromthe surface of said head portion to engage the orthopedic platesurrounding a screw hole and provide rotational resistance when saidbone screw is rotated in a direction of disengagement.
 24. A bone screwaccording to claim 23, wherein: said pawl is resilient.
 25. A bone screwaccording to claim 24, wherein: said pawl is non-radially oriented. 26.A bone screw according to claim 23, wherein: said pawl has a rampedsurface, and when force is presented against said ramped surface, saidpawl at least partially recesses into said head portion of said screw.27. A bone screw according to claim 23, further comprising: a ringincluding said pawl, wherein said head portion includes circumferentialgroove and said ring is rotationally fixed within said groove.